How Asteroid Impacts Could Have Been the Key to Kick-Starting Life on Earth

Scientists have discovered that the impact of comets and asteroids can transform a simple chemical into the letters of our genetic alphabet.

Perhaps the biggest mystery in all of science is when and how life arose on Earth. Was life seeded on our planet from elsewhere by hitching a ride on comets or asteroids, or did it arise spontaneously in the presence of organic molecules? In a new study out today, a team of scientists led by Svatopluk Civiš, a chemist at the Central European Institute of Technology in the Czech Republic, has announced a surprising discovery that blurs the lines between these two theories.

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As they report in the journal Proceedings of the National Academy of Sciences, the impact alone of a meteorite or comet can transform a simple, abundant chemical into the letters of our genetic alphabet—the base pairs that make up our DNA.

The scientists came across this stunning finding by mimicking the energetic collision of an incoming asteroid with a chemical called formamide. This is a simple molecule that is found throughout in the universe and is made with the same four basic atoms as our DNA. Civiš says that while it's been known for years that scientists can coax formamide to recombine into DNA base-pairs, this high-energy spontaneous production of base pairs has never before been seen.

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"It's an incredibly interesting concept," says Raffaele Saladino, an expert in pre-life chemistry at Tuscia University, Italy, who was not involved in the new research. "We normally think of objects like comets as potential carriers of these constituent pieces required for life—not something that's helping synthesize them."

Laser Chemistry

Since scientists can't simply fling meteoroids toward Earth and record the results, they have to simulate large impacts. This team did so using the PALS laser in Prague, one of the most powerful lasers on Earth.

Civiš and his colleagues repeatedly zapped small samples of liquid formamide with a carefully calculated amount of energy, and then analyzed the resulting gas. It may sound strange, but the laser actually produces everything you should expect from such an impact, including a shockwave, a dramatic rise in temperature, and flashes of hard radiation.

When blasted with meteoric levels of laser energy, many of the formamide molecules (which are little more than clusters of carbon, hydrogen, nitrogen, and oxygen) were ripped to shreds. They let loose free-flying atoms and atom pairs, most notably carbon-nitrogen and nitrogen-hydrogen pairs, which latched onto neighboring molecules, quickly building bigger and more complicated chemicals.

In this random atomic slurry, some of the molecules combined and recombined enough times that they formed a few of the letters of the genetic code: guanine, adenine, and uracil. And when the scientists repeated their experiment, but added to their liquid formamide a small amount of clay or ground-up meteroid ("which I purchased off eBay," Civiš says), the process created a fourth letter of the genetic code: cytosine. These additional materials, Civiš says, somehow seem to keep cytosine from continuing to grow and turning into uracil.

Above: The laser spark and dielectric break down of the air on opposite side of the lens. Below: Asterix laser P9030013 at PALS.

Heavy Bombardment

Although you may not have heard of the chemical formamide, in recent years astronomers have discovered that it is seeded throughout the universe, much like pollen. A study in the journal Astrobiology last year reported that not only is formamide found throughout interstellar space, but, as the authors write, "it appears to be a common constituent of star-forming regions that foster planetary systems within the galactic habitable zone."

And given that the molecule stays liquefied above even water's boiling point, Civiš and his coauthors believe it is possible that pools of formamide could have collected as pre-life Earth was bombarded with matter from the solar system.

Furthermore, Civiš says, our earliest records of life on Earth date to a time just after a period of heavy shelling from asteroids and comets that scientists call the Late Heavy Bombardment. While Civiš is hesitant to speculate, he says that it's possible—possible—that this planetary shelling could have formed an abundance of these pre-life molecules, which would help to explain why life arose on Earth when it did.

Even if they're right, the question of how other crucial components of DNA—such as the sugars and lipids that form the spine of our book of life—is still a mystery.